This invention relates generally to sol-gel processes for producing monolithic articles of glass and, more particularly, to processes of this kind that are adapted to eliminate cracking of the gel during a final drying step.
High purity glass components typically are fabricated either by melting solid raw materials or by vapor deposition. Melting solid raw materials is a generally effective technique, but difficulty is sometimes encountered in maintaining purity, due to recontamination from processing containers. In addition, energy costs due to high temperature processing can sometimes be excessive, and finishing costs to produce components of the desired final shapes can also sometimes be excessive. Vapor deposition likewise is a generally effective technique for fabricating high purity glass components. However, it too can be very expensive due to a relatively low material collection efficiency, a high investment cost in processing and pollution control equipment, and slow processing rates.
Research has recently been conducted into the use of a sol-gel process for fabricating high purity monolithic articles of glass. In such processes, a desired solution of glass-forming compounds, solvents and catalysts, i.e., sol, is poured into a mold and allowed to react. Following hydrolysis and condensation reactions, the sol forms a porous matrix of solids, i.e., gel. With additional time, the gel shrinks in size by expelling fluids from its pores. The wet gel is then dried in a controlled environment, to remove remaining liquid from its pores, and it is then densified into a solid monolith.
Advantages of the sol-gel process include chemical purity and homogeneity, flexibility in the selection of compositions, relatively low temperature processing, and the production of monolithic articles close to their desired final shapes, thereby minimizing finishing costs. Nevertheless, the sol-gel process has generally proven to be difficult to use in producing monoliths that are large and free of cracks. These cracks arise during the final drying step of the process, and they are believed to result from stresses due to capillary forces in the gel pores.
Efforts to eliminate the cracking problem during the fabrication of dry sol-gel monoliths have been diverse. In one technique, the gel is dried above the supercritical temperature and pressure of the pore fluid. In another technique, the wet gel body is placed inside a closed container with a few pinholes for venting the evaporating pore liquid in a slow, controlled fashion. In yet another technique, described in U.S. Pat. No. 5,023,208, the pore size of the wet gel is enlarged by a hydrothermal aging treatment prior to drying. The enlarged pore size substantially reduces the capillary stresses generated during drying, so as to substantially reduce the possibility of cracking.
Most of these techniques for eliminating the cracking problem are directed towards manipulating the drying process parameters so as to minimize the capillary stresses, without any particular emphasis on the microstructure of the wet gel and its pristine strength, prior to the start of the drying process. However, if the strength of the wet gel can be increased substantially by tailoring the sol composition and the correct choice of catalyst before the drying process in initiated, it is expected that the gel's resistance to cracking will be much higher. Chances of failure can thereby be minimized substantially. It should be pointed out that, except in the case of the supercritical drying, capillary stresses will always be generated in the gel body irrespective of the drying technique being used. Therefore, an increase of the gel's strength prior to drying usually is advantageous. The gel's strength, of course, is determined by its microstructure.
It also is important to note that the gel's microstructure, in combination with the drying process, has a significant effect on the ability to fabricate large, dry gel monoliths that are free of cracks. For example, it is extremely difficult to fabricate a large, crack-free dry gel structure, e.g., a 3000 cc cylindrical shape, using the supercritical drying process, if the gel has an average pore radius of only 10 Angstroms, even though capillary pressure is non-existent. Gels having an average pore radius on the order of 100 Angstroms, on the other hand, are more suitable for this purpose. Conversely, large, crack-free gels of the same size dried by a pinhole drying process, or any other slow-drying process, are best fabricated by configuring the gels to have smaller average pore radii, e.g., 20 to 30 Angstroms. This is because smaller pore radii impart higher rupture modulus and density to the gel. The gels thereby have a higher probability of withstanding the capillary stress.
It should therefore be appreciated that there is a need for an ability to tailor the gel microstructure so as to obtain desired mechanical and structural properties for the gel prior to the drying process. The present invention satisfies this need.